The present invention is related to a controlling device for a heat-dissipating system and especially to a pulse width modulation (PWM) control circuit for a direct-current (DC) fan.
Please refer to FIG. 1 showing a conventional circuit for controlling the rotation speed of a fan 11 using a transistor 12 as a switch, wherein whether the voltage supply Vcc of DC fan is switched on or not is controlled directly by a pulse width modulation (PWM) signal for changing the rotation speed of fan. However, because the design of such a circuit is to directly control the voltage supply Vcc, some signals output by a frequency generator (FG) and a rotation speed detector (RD), both of which are electrically connected with the fan, are interfered with the PWM signal. As shown in FIG. 2, under the normal condition, the FG signal representative of the rotation speed of fan has the waveform (a) shown in this figure, but it will become as the waveform (c) when it is interfered with the PWM signal having the waveform (b) so that the actual rotation speed of fan is unable to be detected accurately. Likewise, referring to FIG. 3, the RD signal for detecting the rotation speed of fan is at a relatively low voltage level under the normal condition (shown as the waveform (1)) and will be changed from the relatively low voltage level to the relatively high voltage level only under an abnormal condition. Nevertheless, if the voltage supply of a fan is directly controlled by the PWM signal, the RD signal will become as the waveform (3) under the normal condition due to the interference of the PWM signal (shown as the waveform (2)). Because either the FG signal or the RD output signal is interfered with the PWM signal, the actual rotation speed of fan can not be detected out accurately.
In addition, the switched interval of the transistor 12 is changed along with the variation of a duty cycle of the PWM signal to control the average current supplied for the fan 11. However, if it is desired to drive the fan, the duty cycle of the PWM signal input into the transistor must be limited within a certain range enough to allow the fan to rotate, especially more than 30%.
The object of the present invention is to provide a heat-dissipating system with the signals of frequency generator or rotation speed detector immune from interference with the pulse width modulation signal used for controlling the rotation speed of the DC fan.
The present invention provides a controlling device adapted to be used for a heat-dissipating device. The controlling device includes a signal generator electrically connected between the heat-dissipating device and a power source supplying a voltage for the heat-dissipating device, and generating a control signal for switching on/off the power source, and a voltage regulator electrically connected between the heat-dissipating device and the power source and connected in parallel with the signal generator for continuously providing the heat-dissipating device with a basic voltage when the power source is switched off so as to prevent that an output signal of an external device electrically connected with the heat-dissipating device is interfered with the control signal.
The present invention provides another heat-dissipating system. The system includes a power supply, a DC fan, a pulse width modulation circuit generating a pulse width modulation signal, a transistor having a drain connected to the power supply, a gate connected to receive the pulse width modulation signal and a source connected to the DC fan, and a zener diode connected between the power supply and the DC fan, wherein the power supply provides a voltage to the DC fan through the zener diode when the transistor is turned off by the pulse width modulation signal, which prevents the output signal of the external device from interference.
The present invention further provides another heat-dissipating system. The system includes a power supply, a DC fan, a pulse width modulation circuit generating a pulse width modulation signal, a transistor having a drain connected to the power supply, a gate connected to receive the pulse width modulation signal and a source connected to the DC fan, and a resistor connected between the power supply and the DC fan, wherein the power supply provides a voltage to the DC fan through the zener diode when the transistor is turned off by the pulse width modulation signal, which prevents the output signal of the external device from interference.